Vanderburgh County sheriff's deputies found several tanks of anhydrous ammonia at one of the units.
The firefighters began submerging the tanks in water to dissipate them and release the ammonia.
One of them was injured while moving one of the tanks.
The assistant chief of German Township, Dave Bretz, said "These tanks are homemade by people who are manufacturing meth. They're not rocket scientists and they don't really do things right and we had a valve let go on us just dissipated to one of the tanks faster than we wanted."
The firefighter suffered a chemical burn and was taken to Deaconess Hospital, but his injuries were not said to be serious.